The heat cycle of each individual dog and breed varies, but it typically lasts 21 to 28 days. A dog is generally fertile for 18 of those days.
The first heat cycle occurs when a female dog is between 6 and 24 months old, typically occurring earlier for smaller breed animals. An unspayed female has two heat cycles per year, approximately every six months.
The three stages of a female dog's heat cycle include proestrus, estrus and diestrus. During proestrus, the dog's vulva appears swollen and the dog has bloody vaginal discharge and urinates frequently. During this stage, the female appears nervous, distracted and more alert. During estrus, the female is fertile and ready to breed. She instinctively attempts to attract dogs of the opposite sex. The final stage of the heat cycle is diestrus. During this stage, bleeding stops and interest in copulation ceases.